Conductor with insulative layer comprising wood pulp and polyolefin fibers

ABSTRACT

A conductor insulated with a composition of wood pulp and polypropylene or polyethylene fibers is found to exhibit enhanced properties. As compared to wood pulp alone, dissipation factor and dielectric constant are substantially reduced in magnitude and variability, particularly at high frequencies.

United States Patent 1 Jones 4] CONDUCTOR WITH INSULATIVE LAYERCOMPRISING WOOD PULP AND POLYOLEFIN FIBERS [75] Inventor: ThomasBenjamin Jones, Gibson Island, Md.

[73] Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill, NJ.

[22] Filed: July 26, 1974 [21] Appl. No.: 492,247 1 Related US.Application Data [63] Continuation-impart of Ser. No. 359,743, May 14,1973, abandoned, which is a continuation of Ser. No. 921,427, Nov. 24,1970, abandoned.

[52] US. Cl. 174/110 P; 162/138; 174/110 PM;

174/113 R; 174/120 Rv [51] Int. Cl. .Q. H01B 3/48; H01B 7/02 [58] Fieldof Search..... 162/138, 157 R; 174/110 P, 174/110 R, 110 PM, 113 R, 120R 1 Nov. 4, 1975 [56] References Cited UNITED STATES PATENTS 3,385,7525/1968 Selke 162/138 3,401,078 9/ 1 96 8 Grossteinbeck....

3,427,394 2/1969 McKean 174/110 P OTHER PUBLICATIONS The Institute ofPaper Chemistry, Vol. 26, No. 11, p. 920, 6/56, copy 174-110 P.

Primary ExaminerE. A. Goldberg Attorney, Agent, or Firm-C. E. Graves 57ABSTRACT 4 Claims, 7 Drawing Figures r v 1a US. Patent Nov. 4, 1975Sheet 1 of 4 3,917,901

3 300mm mo/EOPm w mmwjom Qz zmmmuw E o mwmEa ozcfma mmjom lNl/E/VTOR 5y7. B. JONES ATTORNEY xom madl m mokusazou WEE m mmE MES U.S. Patent Nov.4, 1975 Sheet 2 0E4 3,917,901

FIG. .3

WOOD PULP+ POLYETHYLENE FIBERS A,B,C,D,EPULP AND POLYETHYLENE FIBERMIXTURES F I0o"/o WOOD PULP 2 E z 2.2- g F 2 CE 6 I 8 A,D PI E J a 0,0 Q

L06 FREQUENCY (Hz) FIG. 4

WOOD PULP POLYETHYLENE FIBERS A,B,c,0,E-PULP AND POLYETHYLENE FIBERMIXTURES F-Io0"/ WOOD PULP F .04

LOG FREQUENCY (HZ) DISSIPATION FACTOR US. Patent Nov. 4, 1975 shw 3 of43,917,901

WOOD PULP POLYETHVLENE FIBERS BRANCHED POLYETHYLENE BINDER @%M@QE MHZ.03

O2 2 A W Q L JE. E I MHZ 0 I o WOOD 0 20 40 6O 00 100/o PULP I00 00 6O40 20 0% PE 2. BPE

COMPOSITION BASED ON WEIGHT Sheet 4 of 4 US. Patent Nov. 4, 1975 FIG. 6

WOOD PULP POLYPROPYLENE FIBERS NO BINDER DENSITY=O.52I

KOBE 29555 Los FREQUENCY (Hz) FIG. 7

WOOD PULP POLYPROPYLENE FIBERS NO BINDER DENSITY= 0.52l

QEDm GE LOG FREQUENCY (Hz) CROSS REFERENCE TO RELATED APPLICATION Thisapplication is a continuation-in-part of my copending application, Ser,No. 359,743, filed May 14, 1973, now abandoned, which was a continuationof my application Ser.,No. 921,427, filed Nov. 24, 1970, also nowabandoned.

FIELD OF. THE INVENTION This invention relates to insulatedcommunications conductors, and more particularly concerns an insulatedconductor with, improved high frequency characteristics over telephonepairs.

BACKGROUND OF THE INVENTION The two principal insulation materials fortelephone conductors in use today are wood pulp and polyethylene. Woodpulp insulation has acceptable dielectric properties. at voicefrequencies, and is hygroscopic. Thus, water entering a pulp-insulatedmultipair cable causes the pulp to swell, which localizes the water atthe fault point. Routine tests based on the electrical discontinuitiescaused by the wet pulp are then employed to accurately locate the fault.However, polyethylene insulation has no wet-swelling property, and henceno built;in mechanism for detection and isolation of water incursions. v

On the other hand, in the increasingly important frequency range from100 kHz to about MHz, the dielectric properties of wood pulp are notfavorable. In particular, the dissipation factor of vpulp produces ahigh component of attenuation due to loss in the dielectric. Thiscomponent is approximately I l .6 dB per mile for a 22 gauge cable pairat 3 MHz compared to 0.3 dB per mile for polyethylene insulatedconductors. Furthermore, with varying temperatures, pulp cables exhibitlarge variations in dielectric loss, capacitance, and capacitanceunbalance to ground. The dielectric losses in pulp also show wide andnonlinear changes in the frequency range of interest.

.These deficiencies make the use of pulp insulated conductors in thetelephone trunk and loop plant unsuitable, for high-frequencytransmission systems such as T2, carrier and visual telephone servicetransmission.

It is well known to form a dielectric layer consisting of wood pulp andpolyolefin fibers on a supportive tape, and to then apply the compositestructure as a wrapping around a conductor. In'the wood pulp wetslurry-insulating process, however, awribbon of wood pulp mat is formedfrom a water slurry onto a wire, without drying, calendaring or, the useof asupporting carrier. In the described preformed tape process,tensilestrength'is supplied by the supportive tape; whereas in thewoodpulp process, .theltensile strength must be derivedfromthe woodpulpitself and from binders where necessary. I Y

The idea of simple addition of polyolefin fibers in the slurry of thewood pulp insulating process is implicitly suggested by the prior art.However, addition of polyolefin fibers of essentially uncontrolled sizewas found to frequently produce an unacceptably weak insulation. In someinstances the insulation also exhibited local electrical anisotropies,limiting the insulation usefulness of the material to low frequencytranflfnission.

POLYOLEFIN For example, relatively long fibers show a marked andunexpected tendency to distribute poorly in they slurry processing onexisting pulp insulating machinery. One consequence is that the fibersdo not occupy avail- 5 able space within the bulk efficiently, andtherefore fail to achieve a uniform distribution. More specifically,relatively long polyolefin fibers tend to segregate themselves from thefiber-pulp slurry and float to the top. Thepolyolefin fibers which arepicked up as insulation .tend to aggregate in clumps.

Accordingly, a primary object of the invention is to develop a materialwith improved dielectric performance at high frequencies.

-.Another object of the invention is to achieve the above object whileretaining the wet-swelling and water permeation properties of pulp thatare useful in localizing and locating water in a cable due to sheathfailure.

A specific object of the invention is to realize an insulated conductorwith wet-swelling, and water permeation properties as well assubstantially lower dissipation factor and dielectric constant in thefrequency range from 100 kHz to 20 MHz.

A further specific object of the invention is to realize efficientpacking of pairs in conductors insulated with the mentioned improvedmaterial.

Another object of the invention is to realize an insulation of thecharacter described in allthese objects, and which has in addition atensilestrengthrelongation property comparable to that of wood pulpalone.

A still further object of the invention is to devise an improvedinsulation material of the character described which is compatible withcylinder-type pulp insulating machinery. I

A specific object of the invention is to develop an insulation havingthe following properties:

low dissipation factor at high frequencies, at least 50% less than pulpat 1 MHz;

- lower dielectric constant than pulp;

minimal variation of dielectric properties with frequency andtemperature;

uniform material properties from lot to lot;

low resistance ,to initial incursion of water for fault location;

water-swelling to create blocking action and fault isolation; and

' capability of being produced from a water slurry and formed directlyand homogeneously on a wire to permit using insulating equipment nowused for pulp.

SUMMARY OF THE INVENTION Pursuant to the invention, composite insulationmaterials including, but not necessarily limited to, wood. pulp withpolypropylene fibers and wood pulp with polyethylene fibers, have beenrealized which exhibit substantial improvement over 100% pulp in highfrequency dielectric properties. They also exhibit the water permeationand wet-swelling features of pulp alone, which are crucial forfault-locating and localizing. Furthermore,- they are adaptable topresent cylinder-type pulp wire insulating equipment. Beneficialreductions in dissipation factor are achieved'pursuant to the invention,by a conductor insulation which is 10 to 40 weight parts wood pulp, to35 weight parts polyolefin and 10 to 25 weight parts binder.

It has been found that, in general, the greater the polyolefin fiber cutlength, the more closely will the tensile strength-elongation productapproach that of quick wood pulp alone. However, it has also been foundthat the greater the fiber cut length, the more poorly the polyolefinfibers disperse in the slurry, and the greater is the incidence ofclumping .of polyolefin fibers within the applied insulation. The clumpsaccount for the referred-to local electrical anisotropies.

Pursuant to a main aspect of the invention, a satisfactory homogeneousinsulation has been achieved, without encountering clumping problems,with a composite wood pulp-polyolefin mixture in which the largemajority of the polyolefin fibers are substantially no longer than 3/4inch and no greater in denier size than 6.

Within this constraint, a satisfactory tensile strength-' elongationproduct has been achieved substantially by the addition of a binderwhich comprises substantially from to weight percent of the totalinsulation on the wire.

The invention," its further objects, features and advantages will befully understood from a reading of the detailed description to follow ofillustrative embodiments thereof.

THE DRAWING FIG. 1 is a schematic perspective of a Fourdrinier papermachine modified for wire insulating purposes;

F1652 is "a schematic cross-sectional view of a wire cluster just'priorto wrapping or polishing the insulating 'ribbonaround the conductors;and

FIGS. 3-7 are graphs depicting typical electricalcharacteristicspertaining to insulated wire made pursuantto'the-inventi'ojn.

DETAILED DESCRIPTION or AN ILLUSTRATIVE EMBODIMENT For purposes ofillustration, the Fourdrinier insulator finiachine schematicallydepicted in FIG. 1, which is of a I type that normally applies pure woodpulp to conductors, is used in the present invention to apply mixturesofwoo d pulp' fibers and synthetic polymer fibers to wires. The teachingapplies equally, however, to the cylinder type wood pulp insulatingmachine of the type described, for example, in US. Pat. No. 1,762,941issued to Edward Wood on June 10, 1930.

A slurry consisting of wood pulp fibers and polyolefin fibers .in wateris prepared and charged into the head box L'Agrdup of wires 2, whichare, for example, 22

gauge copper wires, are fed from a source 2a under a guide roll 2bintothe slurry as it leaves the apron or weir 3, and" onto aconventional Fourdrinier table 4. The latter consists of a continuouswire screen belt 5 mounted on rollers 6, 7. On the screen the insulationis laid down in parallel ribbons formed by channel member 8. Each wirepicks up a coating consisting of a mat of intermingled wood pulp fibersand polyolefin fibers, the result being pictured in FIG. 2.

Uniformity of fiber distribution in the coating is aided by agitatingthe slurry, by addingsmall amounts of surfactants and dispersing agents,and by selecting the" 4 I weight to be described shortly. Then, afterdrying in oven 13, the insulated wires are taken up on spools 14.

In general sulfate wood pulp is suitable for use in the presentinvention and is characterized, typically, by the following:

Tensile strength 2500 psi Alpha cellulose content 8170 (min Alphacellulose plus lignin content 86% (min) Fiber geometry 0.3 by 1.2 m'ils(cross section) Freeness 425 ML (CSF) Aqueous extract conductivity 45micromhos (max) Ash content 0.5% (max) The polyolefin fibersadvantageously are either poly-' ethylene or polypropylene, of thefollowing typical characteristics:

Polypropylene Polyethylene Tensile strength 50,000 psi 50,000 psiElongation 25% 40% Denier size, average 6.0 max 6.0 max Fiber length Y4inch max Y4 inch max Fiber Density 0.93 g/cc 0.91 g/cc To, producesignificant reductions in high frequency dissipation factor anddielectric constant, the amount 1 of polyolefin fiber in the compositematerial as a proportion of the wood pulp-polyolefin totaladvantageously is in the range of 10-90 parts by weight. The moredesired reductions are obtained with 35-90 parts by weight polyolefinfiber.

Beneficial results in terms of dissipation factor, dielectric constantand tensile strength-elongation factor are realized in a conductorhaving an insulative layer of from 10 to 40 parts by weight of woodpulp, from to 35 parts by weight of polyolefin fibers and a bindercomprising from 10 to 25 parts by weight. Within the foregoing, it hasalso been realized that a favorable level of wet-swelling property isachieved when the wood pulp is present in at least about 25 parts.

A dielectric insulative material composed of 67 parts by weight ofpolypropylene fibers and 33 parts by weight pulp when tested at 3 MHz,exhibited a reduction of 83% in dissipation factor (tan 8) and of 37% indielectric constant (e) compared to pulp.

To more precisely determine the electrical properties of insulativematerials as taught by the present inven-' tion, sample sheets A F belowwere made up and tested. I

' c EXAMPLE 1 Several batches consisting of flat wood pulp fibers about0.3 by 1.2 mils in cross section, and flat polyethylene monofilamentfibers 0.2 by 0.8 mil cross sectionallywere mixed in a wate'r slurry inthe proportions by weight shown in Table I below. The binder materialwas Microthene, a low molecular weight, branched polyethylene resin,chosen for its characteristically low melting point. 7

Table I g Percent by weight Thickness Density Sample Pulp PE Binder milsgrams/cc A 15 7s 10 41.5 0.490 B 15 75 10 42.3 0.576 C 25 65 10 42.50.480 D 35 55 l0 43.5 0.497 E 35 55 I0 42.0 0.545

Table I-continued Percent by weight Thickness Density Sample Pulp PEBinder mils grams/cc Prior to sheet formation all fibers were washed andblended in distilled water. Sheets were formed and bonded under pressureat 120C.for 3 minutes, which was sufficient to melt the binder but notthe polyethylene fibers.

The composite sheets so formed were then tested for e and tan 8 atfrequencies of l, 5, I0, 20, 30 and 50 MHz. The variations of e and tan8 with frequency under constant room ambient conditions are shown inFIGS. 3 and 4 respectively.

It is seen that large improvements in magnitude and linearity of e andtan 8 are obtained as the percentage of polyethylene fibers isincreased. Referring to curve C of FIG. 3, the dielectric constant of 3MHz for a 25-65-10% by weight mixture is 1.60 compared to 2.24 for 100%pulp. This is an absolute reduction of about 30%. If an e of 1.0 istaken as a base, the reduction is more than 50%. In the case of tan 8 at3 MHz for the same mixture (see curve C of FIG. 4), a value of 0.006 isobtained compared to 0.028 for 100% pulp. This represents a reduction ofmore than 75%. An even greater improvement is obtained at the higherfrequencies. Additionally, the variation of e and tan 8 with frequencyare much less for all pulp-polyethylene mixtures than for 100% pulp.

FIG. 5 shows the effects of mixture proportions on tan 8 at 1 MHz and 10MHz. The points at 0% pulp were obtained on substantially 100%polyethylene sheets. In general the curves are nearly linear, showingthat tan 8 is closely proportional to the percent by weight of pulpcontained in the composite sheet. The polyethylene fibers and bindershave such a low tan 8 that for all practical purposes they actsubstantially the same as air in determining tan 8 performance whenmixed with pulp.

Dielectric strength tests at 60 Hz showed an average value of about 96volts per mil for the composite sheets, compared to about 130 volts permil for 100% pulp. The dielectric strength does not appear to be relatedto the percent of polyethylene present, but rather to the density of thesheet.

With respect to the specimen sheets of Example I, the electrical testsshowed that insulations composed of mixtures of pulp and polyethylenefibers with a polyethylene binder have high frequency dielectricproperties that improve about linearly with the amount by weight ofpolyethylene present. Thus, a mixture of 50% polyethylene and 50% pulphas a dissipation factor approximately one-half of that of 100% pulp.Improvements of a similar nature are obtained in the case of 6'. Ofequal importance is the great improvement in linearity with frequency ofboth tan 8 and 6 compared to 100% pulp. Due to the similarity inelectrical properties between polyethylene and polypropylene, the samegeneral conclusions as above can be stated for mixtures of wood pulp andpolypropylene fibers. This has been substantiated with tests on variousmixtures of pulp and polypropylene as illustrated in Example 2.

The specimen batches A-E of pulp-polyethylene mixtures, and a specimenof 100% pulp beaten to 530 mil-CFS, were tested for tensile strength,elongation at 6 break and water permeation. The water permeation testsgave a comparative measure of the time required for passage of waterthrough the flat side of the sheets under a head of about one-half inch.The results are shown in Table II below.

The water permeation tests show that the composite sheets do transmitwater, the tests demonstrate the retention by the pulp-polyethylenecomposite of the useful wet-swelling property found in pulp alone.

It is seen that elongation properties of the pulppolyethylene compositesare more favorable than for pulp, which for conductor insulatingpurposes tends to counteract the decrease in tensile strength. It isthis fact which establishes the pertinence of the tensilestrengthelongation factor. Wood pulp alone is highly satisfactory inrespect to this factor. It has been found, how ever, that a tensilestrength-elongation factor down to about half that of wood pulp alone isacceptable as an insulative material covering a wire. The sample I-I-4of Table III on page 14 exemplifies this finding.

EXAMPLE 2 Sheets consisting of 33 parts pulp fibers and 67 partspolypropylene fibers were prepared by a process similar to thatdescribed in Example 1 except that no binder was used. For these sheets,FIG. 6 shows the variation of dissipation factor and dielectric constantwith frequency. The curve for pulp is included for comparison. Largeimprovements in dielectric properties are evident, similar to those forpulp-polyethylene mixtures.

In the blending" process, in general, undrawn fibers are preferred overdrawn fibers although either type may be used. For good dispersion in awater slurry the polyolefin fibers should be precoated with a dispersingagent. Additionally, a small amount (20 drops per 1500 ML of 0.15%slurry) of the agent added to the pulp slurry prior to ,blending hasbeen found highly beneficial. Dispersingagents that have been foundsatisfactory include those known by the trade names Igepal CO-430available from GAF Corporation, and Triton X-l14 available fromRhom-I-Iaas. It further has been found necessary to avoid the formationof air bubbles or vortexes during the blending process, since otherwisethe polyolefin fibers do not disperse uniformly. Thus it was found thatblending by'lateral and vertical agitation is to be preferred tocircular agitation.

To produce an insulation with an adequate tensile strength-elongationproduct, one expedient is to add a binding material to thepulp-polyolefin composite. Preferred binders are a cross-linkable latexcopolymer (Celanese resin CPE-527l an acrylic latex, 45% solids(Rhom-Haas HA12) and a styrene-butadiene emulsion, 50% solids (UniroyalNo. 3595). The referred-to I-IA12 gives good pulp-to-pulp binding andthe No. 3595 gives good polypropylene binding. Blends The CPE-527lbinder provides good adhesion to both pulp and polyolefin fibers. Curingof the binders is accomplished by heating at temperatures of ll 25 C. Ithas been found that the higher the temperature, the shorter the curingtime.

Binders of low melting point polyolefins in powder or emulsions also areuseful. For example, a Microthene branched polyethylene used with heatand pressure gave good results from the standpoint of providing tensilestrength. Powders or emulsions made from amorphous polypropylene havingmelting points less than 125 C. also may be employed. Emulsions areeasier to use than the powders since they will readily mix and remain incontact with the pulp and polyolefin fiber blends. The binders may beadded to the slurry mixtures'prior to formation on the wire or after thepolishing operation on the insulated wire. The latter method,

known as saturation addition is preferred in some situations. A I

A general lowerlimit of substantially any binder of the types mentioned,is approximately parts per 100 weight parts of the composite insulationIn about this amount, the binders contribution to the tensilestrength-elongation product is disproportionally larger than' thecontribution of binder when present in 6 or 8 parts per 100 parts byweight, as can be seen by reference toTable Ill. The binder used forspecimens l-l-l through H-7 was a standard commercial acrylic latexcopolymer.

The tensile strength-elongation data of Table III is normalized .arounda control figure of l00 for wood pulpalone. A lower limit for thisfigure of no less than 50 is workable and acceptable as applied topulppolyolefin-binder composites, as exemplified by the samples l-l-4through H-7. Thus binder parts per 100 by weight of 10, ll, l5, l8 and26 and all values in between are all increasingly acceptable in terms ofthe' resulting-tensile strength-elongation product. From an electricalstandpoint, however, at approximately 25 parts of binder per 100- partsby weight of. composite insulatiomtheincrease in, density and theincrease in lossy material tracing to the binder plus the pulp, wasfound to be so much that the advantage of the polyolefin fibers presentin the composite was canceled. This thus establishesthe upper practicallimit of the binder proportion at about 25 parts per 100 parts by weightof compositeinsulation. Further verification of this general .upperlimit of binder proportion is found in Example 9 later in thisspecification.

The Tablelll data is based on a composite comprising polypropylenefibers of 3 denier size, 4 inch cut length. However, further tests havedemonstrated that substantially the same conclusions just stated willapply 8 if the fibers are polyethylene. Likewise, the same conclusionsapply to fibers within the critical ranges of 1.0 to 6.0 denier and l-/6inch-% inch cut length which are established elsewhere in thisspecification.

EXAMPLE 3 Polypropylene fibers of 3-denier size and l/8-inch long areprecoated with a dispersant. Wood pulp fibers are placed ina blender ina water slurry to a concentration ofO. 15%, to which 20 drops ofdispersant per 1500 milliliters are added. Then, the polypropylenefibers are added to the slurry in amounts sufficient to net a 5050% byweight blend of pulp and polypropylene. During blending, air bubbles andvortexes are avoided by horizontal and vertical agitation rather thancircular. The slurry is piped to the head box (1 in FIG. 1) of aFourdrinier wire insulating machine and passes over the apron to theareas where ribbons are formed. The wires are laid on the ribbons ofinsulation andadvance with the screen 5 to the polishing equipment. Thepolisher wraps the ribbon around the wire in a continuous spiral fashiongiving a continuous uniform insulating coating. The polishing operationalso removes a substantial portion of the remaining moisture in theinsulation. Leaving the polisher, the wires must be damp but not wetwith water. This is important from the standpoint of binder absorption,which is the next step. If the insulation is either too wet or too dry,binder absorption will beimpaired. For this purpose it has been foundthat moisture contents in the range of 40 to by weight are satisfactory.

From the polisher, the wire passes through the binder applicator. Thismay be either. spray or roller-type equipment and is providedwithcontrols of binder concentration and rate of application to yield atakeup advantageously of approximately 20% by weight in the compositeinsulation. Thus when the wire enters the dryer it containsapproximately 40% pulp, 40% polypropylene fibers and 20% binder. Dryingat l 25 C. will cure the binder and provide essentially a moisture-freeinsulation of the above composition. H

A critical lower value of 1.0 on denier s" e and of l/ 16 inch on fibercut length is herein taught because the resulting insulationdemonstrates no tendency to clump and because-as a practical matternosmaller sizes or lengths of fiber are commercially obtainable atpresent. At the upper end, it has been found that a choice of nominalpolyolefin fiber cut length substantially in excess of A inch and/ordenier size in excess of 6.0, yields an unacceptable insulation, as willbe demonstrated below.

Several specimens of wood pulp-polypropylene sheet identified asspecimens G-N in Table IV were prepared having approximately the samedensities and thicknesses but having no binder since for the purpose ofthis example a binder wasnot necessary. The fiber cut length for the twodenier sizes, 3 and 6, was varied in steps from US inch to A inch for3-denier fibers and from% inch to 7/8 inch for 6-denier fibers. In the3- denier specimens G-K, as cut length varies, the tensilestrength-elongation product is seen to go through a minimum of 141 l at3/ 8 inch cut length. At this minimum, the dispersion was found to beoptimum as demonstrated by visual analysis: that is, the greatest degreeof homogeneity of sheet structurewas evident with no clumping ofpolypropylene fibers. As cut length varies either way, the tensilestrength-elongation product improves but at about inche'lectrically'minor but still O, and P did not exhibit clumping.

' Table IV Fiber Product of Ten- Mixture Length sile Strength Sample"7zPulp 7cPP Denier ln. & Elongation G 63 37 3 vs 2337 H 62 38 3 A 1897l 63 37 3 1411 J 62 38 .3 /z 1641 K 62 38 3 A 2512 L 62 38 6 A 2101 M 6238 6 V2 2630 N 62 38 6 "/8 3484' Table V Fiber Product of Ten- MixtureLength sile Strength Sample %Pulp 7cPP Denier ln. & Elongation O 62 38 3V2 1641 P 62 38 6 I V2 2630 O 62 38 8 3796 R 62 38 15 V2 3753 Theexperimental methods used, as well as several examples of experimentswhich led to the above results relating to fiber dispersion, will now becited.

SPECIMEN PREPARATION All samples cited in Tables Ill, IV, and-V weremade by first preparing a water slurry consisting of the desiredproportions by weight of wood pulp fibers and polyolefin fibers. Uniformdispersion of the fibers in the slurry was sought in a magnetic mixerusing a magnetic mixing bar. The mixed slurry was then fed into the bathof an 8 inch X 8 inch Williams sheet mold. Using a quick-release. drain,water was removed and the sheet was formed from thesolids in the slurry.The mixing and blending steps are exactly analogous to those practicedin the standard cylinder type wire insulating process. Each sample wasthen conditioned for about 3 days at 50 percent relative humidity and 72F. preparatory to electrical and mechanical testing.

TENSILE STRENGTH TESTS EXAMPLE 4 A water slurry of substantially equalparts of wood pulp fibers and 15-denier size, 7/8-inch cut lengthpolypropylene fibers was prepared to a medium concentration and mixedmagnetically as described above. Pronounced clumping of fibers at thetop of the slurry was 10 observed visually. The slurry was mixed withagitating action stopping just short of forming a vortex. Still,pronounced clumping was observed. The concentration was diluted by theaddition of A; more water, with no discernible reduction in theclumping. Sheets were prepared as above. The sheets formed withirregularly spaced fiber clumps in large numbers. The forming sheetslost their water very rapidly, creating a sheet with a highly rippled,uneven surface. Rigorous electrical tests were performed to measuredielectric constant on 10 samples, each a 1- /2 inch diameter disc.Discs exhibiting a high degree of clumping had a dielectric constant ofabout 2.0, which approaches the dielectric constant of rawpolypropylene. Discs with low clumping densities exhibited a dielectricconstant of 1.8. The dielectric constant values of 1.8 to 2.0, and thespread of 0.2, are unacceptably high.

EXAMPLE 5 A water slurry of wood pulp fibers and 6-denier size, %-inchfiber cut length polypropylene fibers wasprepared as in Example 4. Theobserved clumping was greatly reduced both in density and clump sizeover Example 4. Addition of A; more water noticeably reduced theclumping further. A minor extent of ripples in the dewatered sheet wasobserved. Dispersion of all fibers on the prepared sheet was, in themain, uniform. Electrical tests were made on 10 1- /2 inch diametersample discs, to determine dielectric constant, yielding readings from1.7 to 1.85. The spread of 0.15 represents at least a substantialimprovement over the 0.2 spread of Example 4.

EXAMPLE 6 greatly reduced size over those of Example 5. Dielectricconstant measurements on 10' l- /z inch diameter discs yielded readingsin a range from 1.58 to 1.65, a spread of 0.07. This range is preferredbecause within it a highly advantageous thin 9-mil thickness ofinsulation on a wire will yield the standard mutual capacitance of 0.083between members of an insulated pair. The spread is also fullyacceptable for reliable high frequency performance.

EXAMPLE 7 A water slurry of substantially equal parts of wood pulpfibers and 3-denier size, A-inch polypropylene fiberswas prepared in theslurry vat of a standard cylinder-type insulating machine/Ribbons wereformed on the cylinder, and then directly dried and tested at a largenumber of points to determine the uniformity of dispersion of the woodpulp and polypropylene fibers, using a differential scanningcalorimeter. The lowest reading reflected the presence of 47 percentpolypropylene fibers, and the highest reflected presence of 52 percentpolypropylene fibers. Most readings were at or much closer to 50percent. These readings reflect a high degree of uniformity indispersion. Electrical tests corroborated that the dispersion uniformitywas more 1 1 than enough. The experiment was continued by formingribbons of the composite insulation around conductors. After drying, thedielectric constant of the thusformed insulation tested out to be 1.6$0.02, a very acceptable situation.

From these data it follows that in general composite pulp-polyolefinmixtures to be used in a water slurry for insulation of wirecharacterized by favorable high frequency properties, should be of anaverage denier size not substantially in excess of 6.0 and of a fibercut not substantially in excess of inch. Polyethylene and polypropylenewere both found to exhibit substantially the same clumping propensity asa function of fiber length and denier size.

The use of fiber cut length of inch or less, and of denier size 6.0 orless is necessary in general as a control on maintaining the homogeneityof the insulation,

over the entire already-mentioned range of parts by weight of to 40 woodpulp, 80 to 35 polyolefin and 10 to binder, within which the beneficiallevels of dissipation factor, dielectric constant and tensilestrength-elongation factor are present. The specific parts by weight ofwood pulp, polyolefin and binder in a given case will be determined by:how great a wetswelling factor is needed, how large a dissipation factorcan be tolerated, how large atensile strength-elongation factor isrequired, how much binder existing mac hinery can tolerate withoutalteration, and other factors which are principally matters ofengineering tradeoff and economics.

To assess the effect of relatively high and low concentrations of binderin a composite insulation of wood pulp, polyolefin and binder, the testsof Examples 8 and 9 were performed.

EXAMPLE '8 A sheet was prepared pursuant to the procedure of Example 4from a slurry of wood pulp and 1/8 inch, '3- denier'size polypropylenefibers. No clumping at the top of the slurrywas observed. Acrylic latexbinder ob- 4O tainable as HA12 from Rhom-l-laas was added to the sheetsuch that the resulting sheet composition when dried was 44 weightpercent wood pulp, 45 weight percent polypropylene fibers, and 11 weightpercent binder. The sheet was examined for fiber dispersion which wasobserved to be favorably uniform with no clumps. Three'samples weretaken from the prepared sheet. Tensile strength, percent elongation andtensile strength elongation product were measured and calculated foreach. Elongation was superior to that of a pulp control sample. Tensilestrength was inferior. The product, averaged for the 3 samples, was 2470as compared to a product of 4300 for the pulp control sample. Theaverage product was acceptable, although for some conductor insulationpurposes more strength would be desirable. Tests for dissipation factorand dielectric constant were made. Dielectric constant over a frequencyrange of 0.001 to 30.0 MHz varied between 1.2666 and 1.238, with a lowoccurring at 1.230. Dissipation factor measured between 0.002 and 0.006.These data compare favorably against corresponding data in FIG. 4 for100% wood pulp.

EXAMPLE 9 Three samples from a sheet were prepared as in Example 8,except that the resulting sheet composition when dried was 36% pulp, 36%polypropylene and 28% binding agent. No clumping at the top of theslurry was observed. The finished sheet was visually examined andexhibited uniform fiber dispersion with no clumping.

Tensile strength-elongation factor measured 6 130,.well

above the characteristic 4300 of wood'pulp alone. Dielectric constantmeasurements were between 1.338 and 1.301, which is acceptable.Dissipation factor, however, at 10 MHz was 0.007, having increased withfrequency very rapidly. The resultant product was judged unacceptabledue to the extreme borderline value of dissipation factor at 10 MHz,tracing in turn to the high percent weight of binder in the product.

It has further been observed that use of polyolefin fibers ofsubstantially A inch or less and of denier size 6.0 or less supply afurther and unexpected advantage of forming a pine-needlelike outerenvelope of protruding fiber ends on the insulation. These outer endswhen later oven dried were microscopically observed to collapse down andtend to mechanically tie up the surface pulp and fibers, thus lendingfirmness to the insulation while still keeping it loose enough tomaintain its wetswelling property. It has been visually andexperimentally observed, however, that this effect is not achieved whenthe fibers are less than about 1/8 inch in length. The use ofpolypropylene fibers of substantially A inch or less and of denier size6.0 or less supply the further and unexpected advantage of forming apine-needlelike outer envelope of protruding fiber ends on theinsulation, which when later oven-dried will collapse down and tend tomechanically tie up the surface pulp and fibers, thus lending firmnessto the insulation while still keeping it loose enough to maintain its"wet-swelling property. This desirable property is realized only when thefibers are not significantly less than l/8 inch in length and 10m deniersize. 1

The spirit of the invention is embraced in the scope of the claims tofollow.

What is claimed is:

- 1. A conductor having an insulative layer comprising from-l0 to 40parts by weight of wood pulp, from to 35 parts by weight of polyolefinfibers and from 10 to 25 parts by weight of a binder, said polyolefinfibers being characterized by a denier size not substantially greaterthan 6.0 and a fiber length not substantially greater than A. inch.

2. The insulated conductor claimed in claim I, wherein the wood pulp isat least 25 parts by weight.

3. The insulated conductor claimed in claim 1, wherein the polyolefinfiber is polypropylene.

v 4. in combination: a conductive element surrounded by-an insulativelayer comprising from 10 to 40 parts by binder, said polyolefin fibersbeing characterized by a fiber length of substantially between l/8* inchand "A inch and by a denier size of not greater than 6.0.".

1. A CONDUCTOR HAVING AN INSULATIVE LAYER COMPRISING FROM 10 TO 40 PARTSBY WEIGHT OF WOOD PULP, FROM 80 TO 35 PARTS BY WEIGHT OF POLYOLEFINFIBERS AND FROM 10 TO 25 PARTS BY WEIGHT OF A BINDER, SAID POLYOLEFINFIBERS BEING CHARCCTERIZED BY A DENIER SIZE NOT SUBSTANTIALLY GREATERTHAN 6.0 AND A FIBER LENGTH
 2. The insulated conductor claimed in claim1, wherein the wood pulp is at least 25 parts by weight.
 3. Theinsulated conductor claimed in claim 1, wherein the polyolefin fiber ispolypropylene.
 4. In combination: a conductive element surrounded by aninsulative layer comprising from 10 to 40 parts by weight of wood pulp,from 80 to 35 parts by weight of polyolefin fibers and from 10 to 25parts by weight of a binder, said polyolefin fibers being characterizedby a fiber length of substantially between 1/8 inch and 1/4 inch and bya denier size of not greater than 6.0.